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UNIT 15 File Processing.

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Presentation on theme: "UNIT 15 File Processing."— Presentation transcript:

1 UNIT 15 File Processing

2 Unit 15: File Processing CS1010 (AY2014/5 Semester 1)Unit15 - 2© NUS Objectives:  Understand the concepts of file I/O  Learn about functions to read and write text files Reference:  Chapter 3, Lessons 3.3 – 3.4  Chapter 7, Lesson 7.4

3 Unit 15: File Processing CS1010 (AY2014/5 Semester 1)Unit15 - 3© NUS 1.Introduction 2.Demo: Sum Array 3.Opening File and File Modes 4.Closing File 5.I/O Functions to Read and Write 5.1Formatted I/O 5.2Detecting End of File & Errors 5.3Character I/O 5.4Line I/O

4 1. Introduction (1/4) CS1010 (AY2014/5 Semester 1)Unit15 - 4© NUS  Problems on arrays usually involve a lot of data, so it is impractical to enter the data through the keyboard.  We have been using the UNIX input file redirection < to redirect data from a text file. Eg: a.out < data1  However, that is not a C mechanism. C provides functions to handle file input/output (I/O).  We will focus on these basic file I/O functions on text files: fopen() fclose() fscanf() fprintf()

5 1. Introduction (2/4) CS1010 (AY2014/5 Semester 1)Unit15 - 5© NUS  In C, input/output is done based on the concept of a stream  A stream can be a file or a consumer/producer of data Monitor Keyboard Hard disk Network port Printer

6 1. Introduction (3/4) CS1010 (AY2014/5 Semester 1)Unit15 - 6© NUS  A stream is accessed using file pointer variable of type FILE *  The I/O functions/macros are defined in stdio.h  Two types of streams: text and binary  We will focus on text stream:  Consists of a sequence of characters organized into lines  Each line contains 0 or more characters followed by a newline character ‘\n’  Text streams stored in files can be viewed/edited easily using a text editor like vim

7 1. Introduction (4/4) CS1010 (AY2014/5 Semester 1)Unit15 - 7© NUS  3 standard streams are predefined:  stdin points to a default input stream (keyboard)  stdout points to a default output stream (screen)  stderr points to a default output stream for error messages (screen)  printf() writes output to stdout  scanf() reads input from stdin  The 3 standard streams do not need to be declared, opened, and closed  There are 2 useful constants in file processing  NULL: null pointer constant  EOF: used to represent end of file or error condition Note that null pointer NULL is not the null character ‘\0’ !

8 2. Demo: Sum Array (1/6) CS1010 (AY2014/5 Semester 1)Unit15 - 8© NUS #include #define MAX 10 // maximum number of elements int scanPrices(float []); float sumPrices(float [], int); void printResult(float); int main(void) { float prices[MAX]; int size = scanPrices(prices); printResult(sumPrices(prices, size)); return 0; } // Compute sum of elements in arr float sumPrices(float arr[], int size) { float sum = 0.0; int i; for (i=0; i

9 2. Demo: Sum Array (2/6) CS1010 (AY2014/5 Semester 1)Unit15 - 9© NUS // Read number of prices and prices into array arr. // Return number of prices read. int scanPrices(float arr[]) { int size, i; printf("Enter number of prices: "); scanf("%d", &size); printf("Enter prices:\n"); for (i=0; i

10 2. Demo: Sum Array (3/6) CS1010 (AY2014/5 Semester 1)Unit © NUS #include #define MAX 10 // maximum number of elements int scanPrices(float []); float sumPrices(float [], int); void printResult(float); int main(void) { float prices[MAX]; int size = scanPrices(prices); printResult(sumPrices(prices, size)); return 0; } // Compute sum of elements in arr float sumPrices(float arr[], int size) { float sum = 0.0; int i; for (i=0; i

11 2. Demo: Sum Array (4/6) CS1010 (AY2014/5 Semester 1)Unit © NUS // Read number of prices and prices into array arr. // Return number of prices read. int scanPrices(float arr[]) { FILE *infile; int size, i; infile = fopen("prices.in", "r"); // open file for reading fscanf(infile, "%d", &size); for (i=0; i

12 2. Demo: Compare Input Functions (5/6) CS1010 (AY2014/5 Semester 1)Unit © NUS int scanPrices(float arr[]) { int size, i; printf("Enter number of prices: "); scanf("%d", &size); printf("Enter prices:\n"); for (i=0; i

13 2. Demo: Compare Output Functions (6/6) CS1010 (AY2014/5 Semester 1)Unit © NUS void printResult(float total_price) { printf("Total price = $%.2f\n", total_price); } Unit15_SumArray.c void printResult(float total_price) { FILE *outfile; outfile = fopen("prices.out", "w"); fprintf(outfile, "Total price = $%.2f\n", total_price); fclose(outfile); } Unit15_SumArray_with_Files.c

14 3. Opening File and File Modes (1/2) CS1010 (AY2014/5 Semester 1)Unit © NUS  Prototype: FILE *fopen(const char *filename, const char *mode)  Returns NULL if error; otherwise, returns a pointer of FILE type  Possible errors: non-existent file (for input), or no permission to open the file  File mode for text files (we will focus only on “r” and “w”): ModeMeaning “r”Open for reading (file must already exist) “w”Open for writing (file needs not exist; if exists, old data are overwritten) “a”Open for appending (file needs not exist) “r+”Open for reading and writing, starting at beginning “w+”Open for reading and writing (truncate if file exists) “a+”Open for reading and writing (append if file exists)

15 3. Opening File and File Modes (2/2) CS1010 (AY2014/5 Semester 1)Unit © NUS  To ensure a file is opened properly, we may add a check. Example: int scanPrices(float arr[]) { FILE *infile; int size, i; if ((infile = fopen("prices.in", "r")) == NULL) { printf("Cannot open file \"prices.in\"\n"); exit(1); }... }  Function exit(n) terminates the program immediately, passing the value n to the operating system. Putting different values for n at different exit() statements allows us to trace where the program terminates. n is typically a positive integer (as 0 means good run)  To use the exit() function, need to include.

16 4. Closing File CS1010 (AY2014/5 Semester 1)Unit © NUS  Prototype: int *fclose(FILE *fp)  Allows a file that is no longer used to be closed  Returns EOF if error is detected; otherwise, returns 0  It is good practice to close a file after use

17 5. I/O Functions to Read and Write CS1010 (AY2014/5 Semester 1)Unit © NUS  Formatted I/O: fprintf, fscanf  Uses format strings to control conversion between character and numeric data  Character I/O: fputc, putc, putchar, fgetc, getc, getchar, ungetc  Reads and writes single characters  Line I/O: fputs, puts, fgets, gets  Reads and writes lines  Used mostly for text streams  Block I/O: fread, fwrite  Used mostly for binary streams  we won’t cover this

18 5.1 Formatted I/O (1/4) CS1010 (AY2014/5 Semester 1)Unit © NUS  Uses format strings to control conversion between character and numeric data  fprintf : converts numeric data to character form and writes to an output stream  fscanf : reads and converts character data from an input stream to numeric form  Both fprintf and fscanf functions can have variable numbers of arguments  Example: float weight, height; FILE *fp1, *fp2;... fscanf(fp1, "%f %f", &weight, &height); fprintf(fp2, "Wt: %f, Ht: %f\n", weight, height);

19 5.1 Formatted I/O (2/4) CS1010 (AY2014/5 Semester 1)Unit © NUS  fprintf returns a negative value if an error occurs; otherwise, returns the number of characters written  fscanf returns EOF if an input failure occurs before any data items can be read; otherwise, returns the number of data items that were read and stored = printf(" … ");fprintf(stdout, " … ");scanf(" … ");fscanf(stdin, " … "); =

20 5.1 Formatted I/O (3/4) CS1010 (AY2014/5 Semester 1)Unit © NUS #include int main(void) { FILE *infile, *outfile; char x; int y; float z; infile = fopen("formatted.in", "r"); outfile = fopen("formatted.out", "w"); fscanf(infile, "%c %d %f", &x, &y, &z); fprintf(outfile, "Data read: %c %d %.2f\n", x, y, z); fclose(infile); fclose(outfile); return 0; } Unit15_Formatted_IO.c File “formatted.in”: What’s the output in “formatted.out”? Data read:

21 5.1 Formatted I/O (4/4) CS1010 (AY2014/5 Semester 1)Unit © NUS #include int main(void) {... if ((infile = fopen("formatted.in", "r")) == NULL) { printf("Cannot open file \"formatted.in\"\n"); exit(1); } if ((outfile = fopen("formatted.out", "w")) == NULL) { printf("Cannot open file \"formatted.out\"\n"); exit(2); }... } Unit15_Formatted_IO_v2.c It is better to check that the files can be opened. Check if file can be opened. To use exit() Use different exit values for debugging purpose.

22 5.2 Detecting End of File & Errors (1/2) CS1010 (AY2014/5 Semester 1)Unit © NUS  Each stream is associated with two indicators: error indicator & end-of-file (EOF) indicator  Both indicators are cleared when the stream is opened  Encountering end-of-file sets end-of-file indicator  Encountering read/write error sets error indicator  An indicator once set remains set until it is explicitly cleared by calling clearerr or some other library function  feof() returns a non-zero value if the end-of-file indicator is set; otherwise returns 0  ferror() returns a non-zero value if the error indicator is set; otherwise returns 0  Need to include

23 5.2 Detecting End of File & Errors (2/2) CS1010 (AY2014/5 Semester 1)Unit © NUS  Caution on using feof() #include int main(void) {... while (!feof(infile)) { fscanf(infile, "%d", &num); printf("Value read: %d\n", num); }... } Unit15_feof.c Value read: 10 Value read: 20 Value read: 30 Output: Input file “feof.in” Why does the last line appear twice? To be discussed in discussion session. (Hint:

24 5.3 Character I/O: Output (1/4) CS1010 (AY2014/5 Semester 1)Unit © NUS  Functions: fputc(), putchar() int ch = 'A'; FILE *fp; putchar(ch); // writes ch to stdout fp = fopen(... ); fputc(ch, fp); // writes ch to fp  fputc() and putchar() return EOF if a write error occurs; otherwise, they return character written

25 5.3 Character I/O: Input (2/4) CS1010 (AY2014/5 Semester 1)Unit © NUS  Functions: fgetc(), getchar(), ungetc() int ch; FILE *fp; ch = getchar() // reads a char from stdin fp = fopen(... ); ch = fgetc(fp); // reads a char from fp  fgetc() and getchar() return EOF if a read error occurs or end of file is reached; otherwise, they return character read  Need to call either feof() or ferror() to distinguish the 2 cases

26 5.3 Character I/O: ungetc (3/4) CS1010 (AY2014/5 Semester 1)Unit © NUS  ungetc() pushes back a character read from a stream and returns the character it pushes back  Example: Read a sequence of digits and stop at the first non-digit int ch; FILE *fp = fopen(... ); while (isdigit(ch = getc(fp))) { // process digit read... } ungetc(ch, fp); // pushes back last char read isdigit(ch) is a function to check whether ch contains a digit character; it returns 1 if so, or 0 otherwise.

27 5.3 Character I/O: Demo Copy File (4/4) CS1010 (AY2014/5 Semester 1)Unit © NUS int copyFile(char sourcefile[], char destfile[]) { FILE*sfp, *dfp; int ch; if ((sfp = fopen(sourcefile, "r")) == NULL) exit(1); // error - can't open source file if ((dfp = fopen(destfile, "w")) == NULL) { fclose(sfp); // close source file exit(2); // error - can't open destination file } while ((ch = fgetc(sfp)) != EOF) { if (fputc(ch, dfp) == EOF) { fclose(sfp); fclose(dfp); exit(3); // error - can't write to file } fclose(sfp); fclose(dfp); return 0; } Unit15_CopyFile.c

28 5.4 Line I/O: Output (1/6) CS1010 (AY2014/5 Semester 1)Unit © NUS  Functions: fputs(), puts() FILE *fp; // writes to stdout with newline character appended puts("Hello world!"); fp = fopen(... ); // writes to fp without newline character appended fputs("Hello world!", fp);  fputs() and puts() return EOF if a write error occurs; otherwise, they return a non-negative number

29 5.4 Line I/O: Input (2/6) CS1010 (AY2014/5 Semester 1)Unit © NUS  Functions: fgets(), gets() char s[100]; FILE *fp; gets(s); // reads a line from stdin fp = fopen(... ); fgets(s, 100, fp); // reads a line from fp  fgets() and gets() store a null character at the end of the string  fgets() and gets() return a null pointer if a read error occurs or end-of-file is encountered before storing any character; otherwise, return first argument  Avoid using gets() due to security issue

30 5.4 Line I/O: fgets() (3/6) CS1010 (AY2014/5 Semester 1)Unit © NUS  Prototype: char *fgets(char *s, int n, FILE *fp)  s is a pointer to the beginning of a character array  n is a count  fp is an input stream  Characters are read from the input stream fp into s until  a newline character is seen,  end-of-file is reached, or  n – 1 characters have been read without encountering newline character or end-of-file  If the input was terminated because of a newline character, the newline character will be stored in the array before the terminating null character (‘\0’)

31 5.4 Line I/O: fgets() (4/6) CS1010 (AY2014/5 Semester 1)Unit © NUS  If end-of-file is encountered before any characters have been read from the stream,  fgets() returns a null pointer  The contents of the array s are unchanged  If a read error is encountered,  fgets() returns a null pointer  The contents of the array s are indeterminate  Whenever NULL is returned, feof or ferror should be used to determine the status

32 5.4 Line I/O: Demo Counting Lines (5/6) CS1010 (AY2014/5 Semester 1)Unit © NUS  Write a function that takes as input the name of a text file and returns the number of lines in the input file.  If an error occurs, the function should return a negative number.  Assume that the length of each line in the file is at most 80 characters.

33 5.4 Line I/O: Demo Counting Lines (6/6) CS1010 (AY2014/5 Semester 1)Unit © NUS #define MAX_LINE_LENGTH 80 int countLines(char filename[]) { FILE *fp; int count = 0; char s[MAX_LINE_LENGTH+1]; if ((fp = fopen(filename, "r")) == NULL) return -1; // error while (fgets(s, MAX_LINE_LENGTH+1, fp) != NULL) count++; if (!feof(fp))// read error encountered count = -1; fclose(fp); return count; } Unit15_CountLines.c

34 Summary CS1010 (AY2014/5 Semester 1)Unit © NUS In this unit, you have learned about How to open text files for reading or writing How to read input from text files How to write output to text files

35 End of File CS1010 (AY2014/5 Semester 1)Unit © NUS


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